1. Field of the Invention
The present invention is in the field of instrumentation and specifically relates to apparatus and method for locating regions in the external wall of a chamber through which a gas, vapor or liquid is leaking.
2. The Prior Art
The helium leak detection technique remains today, many decades after its invention, unequivocally as the best technique ever devised for leak detection, especially for detecting very small leaks or leaks that require quantification in terms of leak rate. The art of helium leak detection is well developed, and the success of the technique results from the ability of helium to penetrate very small passages and from the ability of the mass spectrometer detector to sense extremely small quantities of helium.
The heart of the helium leak detector is the mass spectrometer. A typical mass spectrometer consists of an evacuated chamber into which a gas sample, which may include helium, is drawn. The sample is drawn transversely through an energetic electron beam that flows from a negatively charged filament to a positively charged collector plate. The electron beam knocks electrons from the molecules of the sample gas, whereby the molecules become positively charged ions. These ions are accelerated by an electric field and then passed between the poles of a strong magnet which causes the ions to follow curved paths. The radius of the curved path depends on the mass of the ion and on the strength of the magnetic field. For a particular magnetic field strength, helium ions will pass through a narrow slit in a plate and impinge on a detector, while ions of other masses will miss the slit and will fall on the plate and will not be detected. In this way the mass spectrometer selectively responds to the presence of helium molecules in the evacuated chamber.
In U.S. Pat. No. 4,459,844 issued Jul. 17, 1984 Burkhart describes three major methods of using a helium leak detector to find leaks through the wall of a chamber that is being tested. In the first method, the chamber is placed in communication with a mass spectrometer. Next, the chamber is evacuated and thereafter helium gas is applied to selected regions on the exterior of the chamber. The mass spectrometer operates in high vacuum, and the minute amount of helium leaking into the chamber does not disturb the high vacuum.
In the second method, the chamber to be tested is filled and pressurized with helium and then placed in an evacuated chamber containing a mass spectrometer. The existence of a leak can be discovered, but not its location. The minute amount of helium leaking out of the chamber being tested does not disturb the high vacuum in the evacuated chamber.
The helium leak detector is the leak detector of choice when these first and second methods are used. The vacuum pumps needed to evacuate the chambers also produce the vacuum necessary for operation of the mass spectrometer. The present invention is not intended for use in the first and second methods.
Instead, the present invention is intended for use in what Burkhart refers to as a third method, namely, sampling at atmospheric pressure. In Burkhart's third method, the chamber to be tested is filled and pressurized with helium gas. The chamber remains surrounded by air. A gas sampling probe is moved across the exterior surface of the chamber, drawing a sample of gas through the probe into a vacuum chamber that includes a mass spectrometer.
When the sampling is done at atmospheric pressure, certain shortcomings of the conventional helium leak detector are exposed and the comparative advantages of the present invention come into sharp focus.
The mass spectrometer must operate in a high vacuum; otherwise the filament will burn up and the ions will be scattered from their proper trajectories.
To maintain the necessary high vacuum requires a mechanical roughing pump, a cold trap, and a molecular diffusion pump, along with the ancillary instrumentation. In contrast, the present invention requires none of these cumbersome components.
Sampling at atmospheric pressure compromises the high vacuum required by the helium leak detector. To ameliorate this problem, the volumetric flow rate of the gas sampled must be severely curtailed, resulting in longer transport delays and reduced sensitivity.
As recognized by Gevaud et al. in U.S. Pat. No. 4,294,106 issued Oct. 13, 1981, the slow flow rate of the sampled gas combined with the transport delay of the sample in traveling from the gas sampling probe through a conduit and into the vacuum system implies that the gas sampling probe must be moved rather slowly across the surface of the chamber under test so that the leak alarm signal can be correlated with the position of the probe to permit accurate determination of the location of the leak.
Thus it is seen that the helium leak detector, notwithstanding its superiority when the sample is collected inside a vacuum chamber, has serious limitations for use in a portable system for sampling at atmospheric pressure. These limitations are the need for a high vacuum system and the limited sampling flow rate.
The present inventors recognized that these limitations of the helium leak detector could be avoided if a sensitive, quickly-responding gas detector could be found that does not need to be operated in a vacuum.